This invention relates to a tool break detector and a method of monitoring vibrations produced by machining operations to detect changes in cutting conditions resulting from a broken tool.
Breakage of cutting tool inserts and subsequent damage to the workpiece and/or the machine tool is one of the major problems that must be solved before automation of machining can be successful. Tool breaks that lead to workpiece and machine tool damage must be either prevented by the use of undesirably conservative, low productivity metal cutting rates or must be automatically detected, if close human supervision of the machining process is not available at all times. The automatic tool break detection system should reliably find almost all tool breaks that can damage the workpiece or machine tool, but should not alarm on many other tool breaks that do not significantly affect the machining process, or on any of a variety of potential false alarm sources. Tool break detection needs to be rapid so that tool feed can be controlled to prevent and limit damage.
One class of tool break detectors is based upon the resulting changes in machining power and forces. Another type is based on detecting the acoustic emissions generated by fracture of the tool insert. The current invention employs a different acoustic sensor, such as an accelerometer, and operates in the range of 30-100 KHz, below the usual acoustic emission frequency band.
The machine tool monitor in concurrently filed application Ser. No. 664,189, C. E. Thomas et al, "Acoustic Detection of Tool Break Events in Machine Tool Operations", uses the same sensor and analog signal processing as this invention and avoids false alarms on insignificant tool break events that do not affect cutting conditions. However, that approach assumes that tool insert fracture will be immediately followed by a detectable change in cutting noise or the break event will not affect the workpiece. It therefore generally relies on detecting the tool fracture signal before checking for a cutting noise signal change. The digital pattern recognition circuitry makes a three step check of the processed vibration signal before generating a major tool break alarm. A transient increase in signal level that may have its source in a break event triggers a mean shift detector to check for a change in cutting noise; if the mean shift persists for a given period the alarm is set off. It handles the case of high cutting noise masking the fracture signal by setting the tool fracture or transient detector sensitive enough to frequently trigger the cutting noise change or mean shift detector on high cutting noise peaks. The cutting noise change detector is triggered only on positive-going signal transients.